1. Field of the Invention
The present invention relates to a technique to judge whether a sheet subjected to image formation is in a normal state and to perform control in image forming apparatuses, such as a copying apparatus, a printing apparatus, a facsimile apparatus, and a composite machine of these apparatuses.
2. Description of Related Art
In such an image forming apparatus, printing (image formation) is performed by steps of forming an image on a photoreceptor based on image data acquired by reading a document with a document reading device, transferring the image onto a sheet, and fixing the image onto the sheet.
Conventionally, there has been no measure to check whether images are output normally until the images are output by being transferred onto a sheet. Accordingly, in the case where a sheet is dirty, it is enough to output only one sheet of the dirty sheet as an abnormal image-formed sheet. However, in the case where abnormal images are caused by influences such as dirt (spilled toner, etc.) on a sheet conveyance passage, flaws on a photoreceptor drum and abnormalities in a transferring process, when images are printed on a plurality of sheets, abnormal image-formed sheets are output successively. Further, until an image is printed on a sheet, since the sheet is required to pass through multiple processes, there is a problem that it takes time and labor to detect a portion where abnormalities take place.
For this reason, conventionally, it has been proposed to enable an image forming apparatus to avoid occurrence of abnormalities by detecting transfer unevenness beforehand.
Further, it has been proposed to enable the evaluation of reproducibility with a technique to output information on a difference in image quality onto an operation panel by comparing image data input for image formation with read-out image data acquired by reading an image formed on a sheet.
In addition, with reference to such an image forming apparatus, various kinds of devices have been proposed by each of JPA (Japanese Unexamined Patent Publication No.) 2010-42521 and JPA 2005-331669.
In concrete terms, the above JPA 2010-42521 has proposed to stop a job in the case where image abnormalities continue due to read-out image data. However, although there is a description about control to stop a job, there is no indication of a conception that a job is continued in the case where image abnormalities cause no influence.
Further, the above JPA 2005-331669 has proposed to stop a job if image abnormalities are detected based on read-out image data. However, there is no indication of a conception that a job is continued in accordance with the state of image abnormalities.
Namely, in the above patent documents, if image abnormalities are detected, the execution of image formation is made to stop. Incidentally, in an image forming system in which an image forming apparatus and a sheet processing apparatus are combined, image abnormalities may occur on a region which is finally cut off. However, according to the propositions made hitherto, if image abnormalities occur, image formation is controlled to stop without exception.
Accordingly, even if the situation of a final output matter is no problem, since an image forming apparatus is stopped due to image abnormalities, useless image formation is repeated. With this, useless down time occurs. Further, since image formation is redone, sheets and time are wasted for such image formation, which causes the lowering of productivity.
The present invention has been achieved in view of the above problems, and an object of the invention is to control image formation by detecting image abnormalities appropriately in accordance with the state of an output matter (output sheet, printed matter) at the time of having detected image abnormalities on a sheet by comparing image data used for image formation with image data acquired by reading an image from the sheet.